The present invention relates to a programmable optical fiber thermometer system for accurate measurement of high temperatures.
The accurate measurement of temperatures, and particularly small variations in temperature within high-temperature vessels or furnaces, has long been a desirable object. Many industrial processes require accurate knowledge of such temperatures for process control or for monitoring operations to be carried out within such high-temperature environments. For example, temperatures must be either precisely known or controlled in both jet aircraft turbine engines and within semiconductor diffusion furnaces. In the past, precise temperature control and monitoring has not been possible because the most accurate means of measuring such high temperatures has been the thermocouple. Thermocouples, however, are notoriously inaccurate and fail to provide the precision needed for accurate temperature measurement in regions of very high temperature, that is, between 500.degree. C. and 2400.degree. C. Moreover, such temperatures are often encountered in chemical processes or other environments where the corrosive effects of the superheated materials make the use of a thermocouple sensor impossible.
Pyrometers and other pyrometric devices such as optical light pipes have also been used in the past for measuring temperature. These instruments sense the intensity or wavelength of light generated within the region to be tested and include an output scale which converts this measurement into an equivalent temperature. Such devices are also highly inaccurate because the conversion of light intensity to temperature requires the use of certain assumptions concerning physical parameters governing the emissivity of the test region, and the light collecting and transmitting properties of the pyrometer which do not hold true for all test environments. Also, to attempt to accurately measure the wavelength of the light emitted within the region under test is very difficult due to the wide spectral distribution of light energy at any given temperature.
Another problem relating to pyrometric devices is that the dynamic range of electrical signals generated by a pyrometer can be large. In the past, logarithmic amplifiers have been used to deal with such large variations in electrical signal strength, but such logarithmic amplifiers are extremely inaccurate due to their nonlinear nature introducing electrical measurement errors which are approximations of the true linear conversion of light intensity to electrical signal.
A further shortcoming of pyrometric devices is that there are other multiple sources of error inherent in such devices to impair their accuracy. An optical pyrometer sometimes includes a detector such as a photodiode having an optical filter for filtering out certain wavelengths of light. The bandpass characteristic of the optical filter is, however, dependent upon its ambient temperature. A shift in the ambient temperature will shift the spectral response curve of the filter. Moreover, the detector itself includes a dark current output component which is independent of the flux intensity of the light impinging upon the photodetector. Lastly, the optical path forward of the detector through which the light is transmitted is never a perfect optically-transmissive path. That is, there is some insertion loss between the region where temperature is to be sensed and the detector. Any pyrometer, light pipe, or optical fiber together with associated lenses and mirrors used as an optical transmission line will attenuate the optical signal to some degree.
Some pyrometric devices have attempted to incorporate calibration systems to compensate for such light insertion losses. An example of such a calibration system is shown in Brogardh, U.S. Pat. No. 4,313,344. In Brogardh, a source of light having a known intensity is inserted into an optically-transmissive fiber where it travels to the tip of a blackbody sensor which has been inserted into a region where temperature is to be tested. The blackbody emits light as a function of the temperature and includes a reflecting layer deposited on the tip of the blackbody for reflecting light from the reference source back through the optical fiber. The light from the reference source is modulated so that it may later be separated from the light emitted by the blackbody. Calibration is performed by taking a ratio of the reference signal to the measured signal. The Brogardh system is adequate only in low-temperature environments. In high-temperature environments it is inadequate because the reflectance of the reflecting layer on the tip of the blackbody will change as it is exposed to the high temperature or as the temperature changes. Therefore, the intensity of the reflected light from the nominal light source will begin to vary as a function of temperature and time of operation, and this variance destroys its utility as a calibration signal.
Prior art pyrometric devices and thermocouples have also suffered from inaccuracies in the conversion process, that is, in converting a photodetector output signal into a temperature reading. The conversion process for thermocouples involves interpolation and calibration between fixed points that are known. The conversion for pyrometers should be performed according to the first principle of physics governing the fundamental relationship between temperature and the emission of photons of light energy. This principle is known as Planck's function, and it is nonlinear. In the past, pyrometric devices have attempted to use a linear photodetector coupled with a nonlinear scale calibrated according to certain known temperatures, or have attempted to use nonlinear amplification methods to fit a photodetector output to a linear temperature scale where nonlinear amplifiers approximated a polynominal expansion generated to fit a response curve to link points representing a set of known temperatures. Neither method provides enough accuracy in such devices for measurement of temperatures in high-temperature environments such as the aforementioned semiconductor diffusion furnaces or jet engine turbines. The solution to Planck's function must involve the calculation of a complex integral over all values of the wavelength of the light sensed by the pyrometric device. Measurement accuracy to a degree such as one-thousandth of a degree centigrade requires that this integral be mathematically calculated. Prior art pyrometric systems have not attempted to do this because the inaccuracies inherent in the actual measurement of light intensity have been subject to so many other measurement errors that these errors would have masked any accurate temperature conversion even if such a conversion had been attempted. Moreover, no such system has attempted to directly calculate the integral which defines the response of the system to the Planck function because of the complexity of the mathematics.
Prior art pyrometric systems and thermocouples have also provided little flexibility. Thermocouples have been incapable, for example, of measuring rapid fluctuations in temperature in environments where such fluctuations are common. These may be encountered, for example, in high-speed streams of superheated gases such as those encountered in a jet engine. In such instances it is important for the user to have an instrument which provides an extremely wide frequency response so that these high-speed variations can be measured. Even within the same environment it is occasionally important to know the average temperature without regard for such high-speed fluctuations in temperature. Thermocouples lack this type of frequency response. To date, there have been no pyrometric systems which can provide wide bandwidth, average, and differential temperature measurements in a single unit.
There are currently available fiber optic sensors utilizing a high-temperature optical fiber such as a sapphire rod coupled to a low-temperature optical fiber where the sensor end of the sapphire rod includes a blackbody emitter. The blackbody emitter may consist of an optically-opaque tip sputtered onto the end of the sapphire rod. The low-temperature fiber is coupled to an optical detector having a narrowband filter which provides a notch filter characteristic for the detector, focusing it upon optical wavelengths where relatively small changes in temperature yield relatively large changes in the flux intensity of the light emitted at those wavelengths. This device is described in a paper entitled "High-Temperature Optical Fiber Thermometer" by R. R. Dils, Journal of Applied Physics 54(3), March 1983.